In a dishwasher, the heating of water to be used in the washing process and heating and drying of goods are energy consuming processes. A washer-dryer involves similar processing steps in which water is heated to perform washing of goods, i.e. clothes, and heating and condensation in the following drying step. Generally, the heating of goods and water in a household appliance for washing and drying goods is performed by for example an electric heater heating the water in the washing tub. For performing the drying phase a separate drying system is generally installed, for example by using a hot air source.
In order to save energy, it has been proposed to use a heat pump having a warm side which may be used for heating goods and water and a cold side which may be used for condensing water vapour in a drying phase. In this way a single unit, i.e. the heat pump may be used for two purposes.
In WO2008/053307A1 a dishwasher and a heat pump for dishwashing is described. The described dishwasher comprises a washing portion and a rinsing portion, both arranged in a tunnel. Thus, the dishwasher simultaneously performs a washing cycle in the washing portion and a rinsing and drying cycle in the rinsing portion. The heat pump is provided with a condenser for heating water to the washing portion of the dishwasher and an evaporator for cooling the outlet air from the rinsing portion of the dishwasher. In this way both the warm side and the cold side of the heat pump may be utilized.
However, in most dishwashers the heating of water and goods is earlier in time than the drying phase. Using a heat pump for heating the water in such an appliance has the drawback that the heat generated on the cold side during the water heating phase will not be used and therefore utilization of a heat pump in an energy efficient way for both purposes is not possible.
EP2206824A2 discloses a device comprising a barrel for processing washing goods, and a closed tank filled with liquid. A compressor, a condenser, an expansion valve and an evaporator extract heat from the tank, and supply the heat to the barrel. A sewer pipe discharges process water from the barrel, and a heat exchanger transfers heat from sewage in the fluid in the tank.
Further, in EP2193741A2 a dishwasher is disclosed comprising a heat pump used for heating water in a process area for washing and/or rinsing articles and for accumulating heat in a heat accumulator. A side of a heat coupling tank is joined at the process area and another side of the heat coupling tank is joined at the heat accumulator. A filling medium is provided for program controlled supplying and guiding coupling fluid in or from the heat coupling tank. In the filled state, the heat coupling tank may be used in a drying phase for establishing a cold area onto which water vapour may condense.
A drawback with the above arrangements is that the heating phase for the machine will be prolonged due to the lower effect of the heat pump compared to a conventional heating device. On the other hand, if the heating power of the heat pump is chosen to be the same as a conventional heater, i.e. about 2 kW, such a heat pump will have a size that will be difficult to fit into a domestic dishwasher or washing machine.
In view of the above it may be realized that there is a need for improvements in relation to efficient use of energy in household appliances involving both heating and drying phases.